A demand has arisen in the automobile painting field in recent years for providing paint films with top-coat paints which have a high level of weather resistance, acid resistance and an excellent appearance, and which have storage stability which is adequate for withstanding use with circulation.
Methods involving the inclusion of ultraviolet absorbers and light stabilizers in the paint film are generally used to improve weather resistance, and in terms of the base concealing properties there is little shrinkage on hardening with the addition reaction hardening systems where no eliminated material is produced and the base concealing properties are excellent, and in terms of the acid resistance techniques involving the use of acid/epoxy crosslinking and hydroxyl group/isocyanate crosslinking are known, but with hydroxyl group/isocyanate crosslinking the reactivity at low temperatures is very high and it is impossible to make use of a one-liquid type paint. Furthermore, crosslinking using a blocked isocyanate is also known, but in this case the blocking groups are eliminated at the time of the reaction and the shrinkage on hardening is considerable and adequate base concealing properties are not obtained. Furthermore, in the case of acid/epoxy crosslinking as well the storage stability at low temperature is poor and its use in a one-liquid type paint is limited.
Paint compositions comprising acrylic copolymer obtained by copolymerizing a lactone-modified carboxyl group containing monomer, an epoxy group containing acrylic copolymer and an amino resin have been reported as a technique in which acid/epoxy crosslinking is used (for example, see Japanese Unexamined Patent Application Laid Open H5-171103). However, here again adequate storage stability as a one-liquid type paint is not obtained since an amino resin is used in these paint compositions.
Moreover, paint compositions comprising acrylic poly-carboxylic acid, polyepoxide and antioxidant (for example, see Japanese Unexamined Patent Application Laid Open H9-104803) and paint compositions comprising acrylic polycarboxylic acid resin which contains structural units based on a carboxyl group containing low-lactone-modified hydroxy(meth)acrylate and a polyepoxide (for example, see Japanese Unexamined Patent Application Laid Open 2002-167426) are known, but in these cases again adequate storage stability as a one-liquid type paint is not obtained because the acid/epoxy reactivity is not suppressed.
[Patent Citation 1]
    Japanese Unexamined Patent Application Laid Open H5-171103[Patent Citation 2]    Japanese Unexamined Patent Application Laid Open H9-104803[Patent Citation 3]    Japanese Unexamined Patent Application Laid Open 2002-167426